In cellular mobile radio systems it is fundamental that a mobile station with an established connection on a radio channel should be able to maintain the established connection when moving from one cell served by one base station to another cell served by another base station. The process by which a mobile station maintains an established connection when moving between cells in a cellular radio system is generally called handoff. It is also highly desirous that a mobile station with an established connection on a radio channel be able to maintain the connection when moving within the same cell, even if the radio channel being used is subject to increased interference. If the quality of the established connection falls below specified parameters it is further desirable to automatically disconnect the connection in the event that handoff or other signal processing fails to improve the quality of the connection.
In general, radio communication is only possible when the desired information-carrying radio signals have sufficient signal strength at the receiver and are sufficiently strong in relation to noise and interfering radio signals at the receiver. The minimum strength of course depends on the particular features of the system, e.g., the kind of modulation and receiver used. In order to make sure an established connection may continue on a selected radio channel between a mobile station and a base station, handoff and disconnect processes perform various measurements on radio signals at the intended base and/or mobile stations.
The first cellular mobile radio systems in public use were analog systems for speech or other analog information. The systems comprised multiple radio channels for transmitting analog information between base and mobile stations by transmitting analog-modulated radio signals. These first cellular mobile radio systems had comparably large cells. In general, the signal measurements during the handoff and disconnect processes in such systems were performed by the base stations. One such system is the Nordic Mobile Telephone system NMT 450. Another known cellular mobile radio system, of particular interest as background to the present invention, is the AMPS mobile radio system utilized in the U.S.A.
Recently, digital cellular mobile radio systems for public use have been designed. Digital cellular mobile radio systems comprise digital radio channels for transmitting digital or digitized analog information between base and mobile stations by transmitting digitally modulated radio signals. Digital cellular mobile radio systems may offer substantial advantages over analog cellular mobile radio systems. To achieve these advantages there are certain demands. In particular, channel supervision must be frequent and the handoff and disconnect processes must be rapid and be allowed to be frequently carried out in relation to conventional analog systems.
One digital mobile radio system intended to be a common system for many European countries is the GSM system. In European countries already having an analog cellular mobile system, the new digital GSM system is intended to be introduced as a new system independent of any conventional existing analog systems. The GSM system base and mobile stations are not designed to be compatible with conventional existing systems, but give optimum performance in various aspects by themselves. Accordingly, there has been a comparatively great freedom of choice in technical matters when designing the GSM system. In particular, this is true with the handoff method in the GSM system. In the GSM system, measurements for the purpose of handoff and disconnect are done both by base and mobile stations.
In contrast to the introduction of a new independent digital cellular mobile radio system like the GSM system, in an area with an existing analog cellular system it has been proposed to introduce a digital cellular mobile radio system designed for cooperation with the existing analog cellular mobile radio system. To obtain digital radio channels within the frequency band allotted to cellular mobile radio systems, it is proposed to withdraw a number of radio channels allotted to present analog mobile radio systems and use them in the digital cellular mobile radio system. Due to the proposed design of the digital mobile radio system, three or possibly six digital radio channels may use, in time division multiplex, the frequency band of one previous analog radio channel. Accordingly, replacing some analog channels by multiplex digital radio channels in time division multiplex may increase the total number of radio channels in the joint analog and digital systems.
The intention in this regard is to gradually introduce the digital system and with time gradually increase the number of digital channels and decrease the number of analog channels in the coexisting cellular systems. Analog mobile stations already in use will be able to continue their use of remaining analog channels. New digital mobile stations will be able to use new digital channels. New dual-mode mobile stations will be able to use both remaining analog channels and new digital channels. In such a system, the handoff and disconnect processes for digital radio channels cannot simply be selected to be identical with the handoff and disconnect processes for a strictly digital system, e.g., the GSM system. The handoff and disconnect processes for digital channels must not interfere with the handoff and disconnect processes of the existing analog system and must not require changing of signals in the analog system.
In a dual-mode mobile station, it is desirable to provide the user with the same or comparable functionality in the digital mode as that which is available in the analog mode, since the user is often not aware of whether he is using an analog or a digital voice channel during any particular call. For example, one feature that is available in analog systems is the ability to continue a call without an automatic disconnect even though an interruption of the signal on the channel occurs for a short period of time, such as that which is occasioned by driving through a tunnel or the like. It is desirable to provide this same capability in the digital mode of operation. To this end, it would be further desirable to employ some of the existing equipment in the mobile station, which is used to provide this capability for the analog mode of operation, during the digital mode of operation as well.